This invention generally pertains to methods of injection molding. More specifically, the present invention relates to a method for injection molding which employs fluids of different viscosities.
The invention is particularly applicable to a method by which a relatively viscous fluid, such as a molten thermoplastic, is molded into a particular product with the aid of a relatively non-viscous fluid, such as a gas, during an injection molding process known as gas assisted injection molding. However, it will be appreciated to those skilled in the art that the invention has broader applications and may also be adapted for use in other injection molding environments where both a relatively viscous fluid, such as a plastic or wax, and a relatively non-viscous fluid, such as a gas, steam or a liquid, are injected into a mold cavity.
Gas assisted injection molding processes are becoming widely known in the art. Such processes employ the step of injecting a plasticized (melted) thermoplastic material under high pressure, in the range of 2,000 p.s.i. injection pressure, into a finite mold space to a volume less than 100% of the mold space. Either simultaneously therewith or shortly thereafter a relatively non-viscous fluid such as an inert gas, is injected into the plasticized material in order to fill the remainder of the volume in the mold cavity. The gas which enters the plasticized material moves along the paths of least resistance therein. Such paths are normally in areas where the thermoplastic body is the thickest and has slower cooling sections. In this way, with a suitably designed part, one or more hollowed out sections can be provided in the part. The material is displaced by the gas from the middle of these sections and moves out to fill the remainder of the mold space. In this way, the plastic material remains held against the mold surfaces during hardening and sink takes place internally, rather than on the exterior surfaces of the part. Since the pressure used for final filling of the part is confined to an area defined by the gas channel or cavity, the resultant force against the sections of the mold is relatively modest so that lower clamping forces on the mold are adequate.
The added equipment and process control mechanisms necessary to implement gas assisted injection molding contributes significantly to the cost and complexity of this type of molding process. The circuits needed to charge, inject and vent the pressurized gas at specific times and at desired pressures are quite complex and the methods for utilizing such apparatus have not at this point been optimized. Another problem with conventional injection molding processes in general is the venting of the gas from the gas cavity formed in the molded part. A further problem is that the time required to cool the molded product is substantial in relation to the time that the injection molding process itself takes. Thus, producing a quantity of such products is a time-consuming process.
Accordingly, it has been considered desirable to develop a new and improved injection molding process which would overcome the foregoing difficulties and others while providing better and more advantageous overall results.